Low pressure / prop valve switch... HELP !!!!

[72stepside]

I'm almost finished with my disc brake conversion. Got everything bled tonight and found a leak. It is THROUGH the lowpressure switch on the prop valve. The fluid actually comes up through the top of the switch where the electrical connection is.

I thought I had a bad switch so I took it out and replaced it with the one in the old distribution block THAT HAD NEVER LEAKED. It looked exactly the same.

IT FREAKIN' LEAKED THROUGH THE TOP OF IT TOO !!!!

If it matters, the little "nipple looking thing" on the front of the prop valve is sticking out farther than when I installed it. The brake pressure appears to have pushed it out. When connected to the wiring harness, the brake light indicator in the cab does not light up.

Anybody got any ideas?

What is the formal name of this switch? I can't find one on autozone.com

[kid]

The valve must be centered when the switch goes in or it shoves the toggle arm of the switch through the switch when it is screwed down.

Brake warning sensor is the proper name.

Oh and it is not a proportioning valve it is a distribution block with warning switch. Proportioning valves are used on single master cylinder disc brake drum brake cars like 66 and earlier Corvettes. Our Proportioning system is the dual piston Master cylinder. The size of the pistons determine the ratio or proportion of the pressure.

http://www.lmctruck.com/icatalog/fb/0118.asp item 5

Not carried at Autozone or Checker.

[MadMaxetc]

[quote="kid"]...Oh and it is not a proportioning valve it is a distribution block with warning switch. Proportioning valves are used on single master cylinder disc brake drum brake cars like 66 and earlier Corvettes. Our Proportioning system is the dual piston Master cylinder. The size of the pistons determine the ratio or proportion of the pressure...[quote]

Good to know. I guess I have been calling it the wrong thing for years!

[72stepside]

Here's my theory:

The switch is not "fluid tight". With the switch off, you can blow air through it, open or closed. The electrical fitting on the pigtail has what appears to be a rubber seal in it. This prevents the brake fluid from running out the top. When I took the pigtail off, I broke a 30 year old seal which I could never regain, hence the leak.

In my case, the low pressure sensor was slid over, engaging the switch which caused the light to come on (it didn't come on at first as I mentioned in the post above).

So, I loosened the bleeder valve on the rear and hit the pedal to move the low pressure sensor back to the center which turned off the light. I checked with the sensor centered to see if fluid would leak through the switch. It did of course. I went to the parts house and got a plug to put in place of the switch to stop the leak. The indicator light will not work now but, due to this being a dual system, I can live with it. If there was only a one resevoir master cylinder, I wouldn't be doing this.

So, I guess the moral of the story is to not remove the electrical socket from the low pressure switch when removing or installing a proportioning valve / distribution block.

[kid]

The switch is fluid tight to a degree that the leakage on the shuttle that does the switching does not overpower the little seal. If it is screwed in out of alignment then the seal and switch are broke. If a new switch is put in with the shuttle in the right place then you can check for a major cause of leakage. Dot 3 absorbs water and rust our cylinders it also rusts the shuttle and causes excess leakage from side to side and out the switch. My warning light came on and went off one day and two days later I was rebuilding the back brakes. Pass side went metal to metal, so, I will never get rid of mine.

[flyboy2610]

So, I guess the moral of the story is to not remove the electrical socket from the low pressure switch when removing or installing a proportioning valve / distribution block.

Good tip. If it ain't broke, don't fix it.